Fuel burner control system



Feb. 8, 1966 R. w. DE LANCEY ,434

FUEL BURNER CONTROL SYSTEM 3 Sheets-Sheet 1 Filed May '7, 1959 Feb. 8, 1966 R. w. DE LANCEY FUEL BURNER CONTROL SYSTEM 5 Sheets-Sheet 2 Filed May '7 1959 Feb. 8, 1966 R. w. DE LANCEY FUEL BURNER CONTROL SYSTEM 3 Sheets-Sheet 3 Filed May 7 1959 FS'mOEmWIF United States Patent 3,234,434 FUEL BURNER CONTROL SYSTEM Ralph W. De Lancey, 11809 Goshen Ave., West Los Angeles, Calif. Filed May 7, 1959, Ser. No. 811,617 2 Claims. (Cl. 317-132) This invention relates to electrical control devices and more particularly to primary controls for fuel burners, such as oil burners and the like.

It is a principal object of the invention to provide a new and improved fuel burner primary control.

It is a further object of the invention to provide a fuel burner primary control which performs all of the essential control functions and yet is simple, compact and inexpensive to manufacture.

Another object of the invention is to provide a new and improved fuel burner primary control which is well adapted for use with a wide variety of furnace installations.

Still another object of the invention is to provide such a new and improved fuel burner primary control of such construction as to facilitate installation and to provide ready access for service or repair.

Yet another object of the invention is to provide a new and improved fuel burner primary control which may be constructed in any of avariety of different models to suit various furnace installations, as for example with either built-in or remote combustion detector switch, and either with or without automatic recycling, while maintaining maximum standardization of parts.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The organization and manner of operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which:

FIGURE 1 is a fragmentary perspective view, partly in cross-section of an illustrative embodiment of the invention as installed upon the stack of a furnace provided with a gas or oil burner, the furnace and the burner'being omitted from the drawing;

FIGURE 2 is a fragmentary perspective view, partially cut away, to show the internal construction of the primary control shown in FIGURE 1;

FIGURE 3 is a cross-sectional view taken along the line 3-3 of FIGURE 1;

FIGURE 4 is a fragmentary cross-sectional View, taken along the line 44 of FIGURE 3;

FIGURE 5 is a fragmentary cross-sectional view taken along the line 55 of FIGURE 2;

FIGURE 6 is an exploded perspective view illustrating the construction of a portion of the combustion detector switch of the primary control shown in the preceding figures;

FIGURE 7 is a plan view, partly in section, illustrating the operation of the combustion detector switch actuator shown in FIGURE 6; and

FIGURE 8 is a schematic wiring diagram of the burner control system shown and described in connection with the preceding figures.

FIGURE 1 shows a primary control 10 comprising a housing 11 to the base of which is fixed a radiant .heat shield 12. A stack tube 13 extends from radiant heat shield 12 into the exhaust stack 14 of a furnace (not shown) equipped with a gas or oil burner (not shown). The entire unit is supported in position by means of a stack flange 15 which is secured to stack tube 13 by means 3,234,434 Patented Feb. 8, 1966 ICC of a set screw 16 and which is screwed or bolted to exhaust stack 14, all in conventional manner.

As shown in FIGURE 2, housing 11 is fabricated of resilient sheet metal material, such as aluminum alloy or the like, and includes an end wall portion 17 and a plurality of integral side wall portions 18, 19, 20 and 21 respectively. A pair of these integral side wall portions, 18 and 19, extend generally perpendicularly from opposite sides of end wall portion 17. The other pair of side wall portions, 20 and 21, also extend generally perpendicularly from the intervening pair of opposite sides of end wall portion 17, and are provided wtih cut-outs 22 and 23 respectively, each affording access to the interior of housing 11. Side wall portions 20 and 21 are each provided with a pair of indexing apertures 24-only one of each pair of indexing apertures 24 is visible in FIGURE 2 which are spaced from end wall portion 17 by a distance greater than cut-outs 22 and 23 respectively.

The primary control of the present invention includes a chassis member 25 of insulating material, such as pressed fiberboard or Bakelite, which is provided with a plurality of laterally extending tabs 26 adapted to be received in indexing apertures 24 of side wall portions 20 and 21 of housing 11. Chassis member 25 supports a plurality of interconnected electrical components including a transformer 27, a relay 28, a safety switch 29 and a combustion detector switch 30, through the use of mounting brackets 31 and 32 which are bolted or riveted to chassis member 25. Brackets 31 and 32, and the manner in which components 27-30 are mounted thereon, are more clearly visible in FIGURE 3; the construction is such that the major components of the primary control may first be assembled to mounting brackets 31 and 32, and the resulting subassenrblies simply and conveniently riveted or otherwise secured to insulating chassis member 25.

Chassis member 25 also includes a pair of opposite edge portions 33 and 34 which serve as terminal strips and are provided with terminals 35 and 36 respectively to which external circuit connections may be established. Chassis member 25, with the major circuit components installed thereon, is inserted within the housing by simply snapping laterally extending tabs 26 into indexing apertures 24 of side wall portions 20 and 21; side wall portions 18 and 19 of housing 11 are preferably de-bossed as shown at 37 in FIGURES 1 and 2 to further receive index, and support chassis member 25.

The unit is provided with a removable metal cover plate 38 adapted to be secured to housing member 11 to cover cut-out 22, and cover plate 38 is provided with an aperture affording access for external wiring to terminals 35. In FIGURE 1, a rubber grommet 39 is seated in this aperture and itself is centrally apertured to admit the usual wire from a conventional room thermostat (not shown). When installing the primary control of the present. invention, cover plate 38 is removed by loosening screws 40 and 41, sliding cover plate 38 away from the stack and lifting it from the housing 11, and then inserting the thermostat wire through grommet 39 and making the connections to terminals 35 before replacing the cover plate 38.

A similar metal cover plate 43 is provided on the bottom surface of housing 11 to cover cut-out 23 in side wall portion 21 and to admit and facilitate the establishmerit of the required power line connections, and the connections to the conventional motor and ignition device. To this end, cover plate 43 is preferably provided with a pair of apertures each adapted to receive a conventional electrical wiring conduit fitting (not shown).

When the wiring has been completed, and metal cover plates 38 and 43 installed in position shown in FIGURE 1, the protective enclosure for the primary control is completed by slipping a U-shaped sheet metal cover member 44 over the front end of the housing 11. Cover member 44 is equipped with a latch member 45 which is centrally pivoted at 46 and provided with a forwardly extending latch operating tab 47 projecting through a slot 48 in cover member 44. Latch fingers 49 at the ends of latch member 45 are adapted to engage or mate with inwardly extending catch elements 50 which are integral with side wall portions 18 and 19 of housing 11, upon moving latch actuating member 47 downwardly; the mechanism is shown in the latched position in FIG- URES 1 and 2. To permit locking the cover in position, a locking screw -1 extends through an arcuate slot 52 in cover member 44 and is threaded into latch member 45.

With metal cover plates 38 and 43 and U-shaped cover 44 in position, the unit provides a neat and trim appearance while at the same time completely enclosing the control apparatus mounted on chassis member 25.

The combustion detector switch 30, in a unit of the type herein illustrated which provides for automatically recycling in the event of momentary power failure, comprises three pairs of contacts 60, 61 and 62, as most clearly shown in FIGURE 5. These contacts are controlled by a switch actuator 63 (FIGURE 2), more fully to be described in connection with FIGURE 6, which is clutch-mounted on a rotatable shaft 64 passing through chassis member 25, end wall portion 17, radiant heat shield 12, and the interior of stack tube 13. The remote end of shaft 64 is slotted to receive one end of a helically formed bimetallic thermally sensitive element 65 (FIG- URES l and 3), the other end of which is fixed to stack tube 13 as by means of a screw 66 in a conventional manner. Shaft 64 is retained by means of a pair of snap rings 67a on opposite sides of chassis member 25, rings 67a engaging appropriately spaced grooves (not shown) in shaft 64. Temperature increases within stack 14, which accompany combustion in the furnace, cause bimetallic element 65 to rotate shaft 64 in a clockwise direction .as Viewed in FIGURE 2; conversely, shaft 64 is rotated counterclockwise in response to falling temperture within stack 14.

On the forward end of shaft 64, as viewed in FIGURE 2, is mounted the combustion detector switch actuating assembly 63, the construction of which may be more clearly visualized from the exploded perspective View of FIGURE 6. At the forward end of shaft 64 is provided a square sharft section 67 to which are keyed a pair of metal clutch plates 68 and 69. Between plates 68 and 69, in the order named, are provided a clutch bearing member 70, a clutch anrnbearing members 71, a first clutch arm 72, another clutch arm bearing member 73, a second clutch bearing member 74, a third clutch arm bearing member 75, a second clutch arm 76, a fourth clutch arm bearing member 77, and a third clutch bearing member 78. Clutch bearing members 70, 74 and 78 are keyed to square shaft portion 67, while the remaining elements are centrally apertured to permit freedom of rotation with respect to square shaft portion 67. Members 70, 71, 73, 74, 75, 77, and 78 are each constructed of plastic material such as polytetrafluoroethylene, currently available on the market under the trademark Teflon. Clutch arms 72 and 76 are constructed of insulating material, such as Bakelite or pressed ,fiber board. Clutch arms 72 and 76 are provided with hub portions, 79 and 80 respectively, adapted to be received Within the central apertures of clutch arm bearing members 71, 73 and 75, 77; this provides for easy assembly and insures proper clearance about square shaft portion 67 as well as accurately controlled clutch action. The entire assernbly is mounted on square shaft portion 67 of stack switch shaft 64 and is retained by means of a snap ring 81 which engages a groove 82 in the forward end of shaft 64, anteriorly of square shaft portion 67. The entire assembly is maintained in compression by means of a clutch spring 83 which bears on clutch plate 69 and on a retaining guide member 84 fixed to shaft 64. Clutch arm bearing members 71, 73 and 75, 77 are keyed to clutch arms 72 and 76 respectively by means of projections 85 from the clutch anm bearing members which engage mating apertures 86 in the clutch arms.

Clutch arms 72 and 76 are provided with respective switch actuating members 87 and 88 adapted to operate selected contacts of stack switch 30. In the illustrated embodiment of the invention, which provides for automatic recycling in the event of momentary power failure, the forward clutch arm 72 is also provided with a vpair of projections 89 and 90 defining a channel 91. A generally U-shaped bracket 92 is secured to the rearwardly disposed clutch arm 76 and its forward projection is provided with a slotted guide aperture 93 which receives a recycle adjustment arm 94 adapted to extend through channel 91 between projections 89 and 90 of clutch arm 72, to constitute a lost-motion connection between clutch arms 72 and 76. Arm 94 is pivotally mounted to bracket 92 as indicated at 95. For purposes of standardization of parts, clutch arm 76 may be constructed identically to clutch arm 72, although in the case of clutch arm 76, the elements corresponding to projections 89 and 90 and channel 91 of clutch arm 72 are non-functional. To prevent inadvertent variation in the setting of recycle adjustment arm 94, arm 94 and arcuate guide slot 93 may be formed to provide a detenting function, as is clearly visible in FIGURE 7.

Combustion detector switch 30 itself may be of conventional stacked construction in which the switch blades bearing the contacts are secured between insulating spacers. In addition to the switch blades, however, three heavier snubber blades 97, 98 and 99 are included in the stack assembly. Blades 97 and 98 are the outermost blades in the assembly, and blade 99 is interposed between contact pairs 60 and 61 (see FIGURE 5). The uppermost blade 101 of contact pair 61 projects beyond the other blades in the assembly for engagement by actuating member 87 of clutch arm 72. Actuating member 88 of clutch arm 76 is disposed between snubber blade 99 and the lower blade 100 of contact pair 60. Snubber blades 97, 98 and 99, while stiffer than the switch blades themselves, are also somewhat resilient, but snubber blade 99 is sufficiently stiff to resist downward pressure applied through actuating member 88; in other words, contact pairs 61 and 62 can be actuated only by downward pressure on switch blade 101 by actuating member 87 of clutch arm 72. Snubber blades 97 and 98 are sufficiently stiif to result in slippage of the clutch mechanism of FIGURE 6 by permitting only restricted rotation ofthe clutch arms -76 and 72.

The lock out mechanism including safety switch 29 is more clearly shown in FIGURE 4. Safety switch 29 includes a normally closed contact pair 110. The actuating mechanism for safety switch 29 includes a thermally sen sitive bi-metallic actuator 111 which is fixed at one end and passes through the core of a resistance heater 112 so that its free end normally bears downwardly on the upper blade of contact pair to maintain the contacts in closed position. Resistance heater 112 is preferably constructed to have a substantial positive temperature co-efficient of resistance, and to this end may be constructed of nickel wire or the like mounted in a ceramic form. On heating of bi-metallic switch actuator 111,

the lower switch blade.

silicone fiber glass) directed toward bi-metal lic actuator 111. A heat shield 115, of stainless steel or the like, is interposed between compensator element 113 and resistance heater 112, so that compensator 113 is affected only by variations in ambient temperature. Bi-metallic elements 111 and 113 are reversed with respect to each other; the low expansion side of element 111 is on the left as seen in FIGURE 4, while the low expansion side of compensator element 113 is on the right as seen in the same figure.

Ambient temperature compensating element 113 opposes the actuation of safety switch 29 by bi-metallic actuator 111, and the degree or amount of such opposition is dependent upon the ambient temperature to which the control unit is exposed. The position of heat shield 115 between compensator 113 and resistance heater 112 renders compensator 113 responsive only to variations in ambient temperature, and such variations result in movement of compensator tip 114 with respect to bimetallic actuator 111. Thus, it takes a greater temperature rise to deflect element 111 beyond the end of the upper switch blade of contact pair 110 at a higher ambient temperature than at a lower one, due to the greater opposition or resistance offered to deflection of element 111 by compensator tip 114 at the higher ambient temperature. This tends to stabilize or equalize the performance of the unit against variation in ambient temperature from one furnace installation to another.

To provide for resetting of the safety switch 29, an insulating rod 116 of Bakelite or the like extends through mounting bracket 32 and is captivated therein as bymeans of a snap ring 117. The innermost end of rod 116 is 'formed to provide a tip 118 which extends through a hole in the upper switch blade of contact pair 110'to bear on The outermost end 119 of rod 116 projects through a hole in the outer housing 111 to constitute a reset button. The reset button, when depressed, depresses first the lower and then the upper switch blade of contact pair 110, thus permitting actuator member 111 to snap back into its normal position as illustrated in FIGURE 4, assuming that suificient time has elapsed since the operation of the safety circuit to'permit cooling of bi-metallic actuator element 111.

Transformer 28 may be of conventional construction, having a 115-volt primary winding adapted to be connected across a conventional AC. power supply, and having a tapped secondary winding with the tap preferably being situated to provide an output voltage of substantially /3 of the total secondary output voltage. In a specific embodiment of the invention, the full secondary winding may be constructed to provide an output voltage of 17 /2 volts, and the tap may be situated to provide a second output voltage of about 5/2 volts with respect to one of the terminals of the secondary winding.

Relay 28 may be of entirely conventional construction, I

being provided with two normally open contact pairs 120 and 121 adapted to be closed upon energization of a solenoid winding having an iron core.

The components of the primary control are connected in a circuit such as that shown in FIGURE 8; much of the wiring has been omitted in the preceding structural figures to facilitate the illustration of the operating elements of the device. In FIGURE 8, the primary winding of transformer 27 is connected to a pair 36A and 36B of the lowermost terminals 36 within the primary control housing 11. A third terminal 36C affords access for connection of the motor 130 and ignition device 131 in a conventional manner. One of the contact pairs. R1 of relay 28 is connected between terminals 36A and 36C. A fan and limit switch 132 of conventional construction is also provided in most normal installations.

The upper terminals 35A and 35B of the primary control are connected to the room thermostat of any normal installation. The common terminal of the secondary winding of transformer 27 is connected to terminal 35A through normally closed contact pair of safety switch 29. The other terminal of the secondary winding of transformer 27 is connected through resistance heater 112 and the lowermost contact pair 62 of stack switch 30, thence through the solenoid winding R of relay 28 to terminal 35 B. The tap 133 on the secondary winding of transformer 27 is connected through contact pairs 60 and 61 of stack switch 30, which contact pairs are connected in parallel with each other, and through the second normally open contact pair R2 of relay 28, thence through the solenoid winding R of relay 28 to terminal 35B.

Operation In starting a cold furnace, contact pairs 61 and 62 of stack switch 30 are closed and contact pair 60 is open. When the room thermostat calls for heat, the secondary circuit of FIGURE 8 is completed and the full secondary voltage, less a small drop across resistance heater 112 which may have a cold resistance of only about 19 or 20 ohms, is impressed across the solenoid winding R of relay 28. This pulls in the relay, closing contact pairs R1 and R2, whichstarts the motor 130 and energizes the ignition device 131. If the burner ignites immediately, as it does under normal conditions, the temperature in stack 14 rapidly increases and shaft 64 is rotated in a clockwise direction as viewed in FIGURE 2. This causes clutch arms 72 and 76 also to rotate in a clockwise direction until contact pair 60 of stack switch 30 is closed, this contact pair constituting the hot contacts. As the heat in stack 14 continues to increase, actuator 88 of clutch arm 76 is restrained by snubber blade 97, but actuator arm .87 of clutch arm 72 continues to rotate in a clockwise direction until projection 89 encounters recycle adjustment arm 94, at which point further rotation is restrained. This action, which can best be visualized from FIGURE 7, results in opening contact pairs 61 and 62 and disrupts the flow of current through resistance heater 112. If this takes place in less than about 90 seconds, for normal ambient temperature of 70 and normal line voltage of volts, the current through resistance heater 112 is disrupted before actuator 111 (FIGURE 4) is deformed 'sufiiciently to open safety switch contact pair 110.

Relay 28 is maintained energized. by the tapped secondary current passing through its own holding contacts R2 and whichever one of contact pairs 60 and 61 is closed, so long as resistance heater 112 is not sufliciently heated to open contact pair 110; the arrangement is preferably such that contacts 60 make before contacts 61 and 62 break. The tapped secondary voltage is incapable, however, of energizing relay 28 since in the de-energized condition relay contacts R2 are open.

When the room thermostat contacts open, solenoid R of relay 28 is de-energized and relay contact pairs R1 and R2 are immediately opened. This cuts out motor and de-energizes ignition device 131 to shut down the furnace. As the temperature in stack 14 cools, shaft 64 rotates in a "counterclockwise direction due to cooling of thermally sensitive element 65 (FIGURE 1), and actuating member 88 of clutch arm 76 moves downwardly as seen in FIG- URE 7 to open hot" contacts 60. Actuator 88 is then restrained by snubber blade 99, but with further cooling of stack 14 actuator 87 of clutch arm 72 continues to move downwardly as seen in FIGURES 5 and 7, until it engages switch blade 101 (FIGURE 5) and, with continued rotation, first closes contact pair 61 and then contact pair 62. Until contact pair 62 is closed, the system is not conditioned for re-ignition.

The time lag required for closing of contact pair 62 is determined by the adjustment of re-cycling adjustment arm 94 in arcuate slot 93, the maximum time lag being vdetermined when arm 94 is in its extreme right-hand position as seen in FIGURE 7 and the minimum recycling time when arm 94 is in its extreme left-hand position. This permits ready adaptation to a wide variety of different furnace installations.

The operation of the recycling system is as follows: In the event of momentary power failure, relay 28 automatically and immediately drops out contacts R1 and R2, shutting down the furnace as described above. Contacts 61, when closed, sets up a relay holding circuit. Should contacts 62 make and then open again quickly, relay will remain energized. When contacts 62 subsequently are closed by arm 87 of clutch arm 72, if the thermostat contacts remain closed demanding heat from the furnace, the system proceeds through a normal start cycle. If the momentary disrupting condition has corrected itself, the burner will re-ignite automatically; if not, the safety switch will operate to lock-out the relay, as described above.

In the event of flame interruption, the temperature in stack 14 drops, and contact pair 62 is closed by actuator 87 as the stack cools. If re-ignition is not established, the safety switch 29 operates to lock-out the relay R.

If for any reason the burner should fail to ignite on a normal start cycle, the continued flow of current through resistance heater 112 brings about two results. As resistance 112 heats up, its resistance increases due to its positive temperature coefiicient of resistance. With a nickelwire resistor, the cold resistance of 19 ohms at normal ambient temperature of 7 F. increases to 30 or 35 ohms at 300 to 350 degrees, and this reduces the voltage across the coil R of relay 28. The parameters may be adjusted so that this action itself causes the relay to drop out; the bimetal safety switch opens the circuit allowing the resistor to cool so that the control can be reset in the normal manner. Without the bimetal safety switch, the circuit must be manually opened to allow the positive temperature coeflicient resistor to cool before the control can be restarted.

In the event that the resistance increase of resistor 112 does not cause the relay to become de-energized, heating of resistance 112 deforms bi-metallic actuator 111 to open safety switch contact pair 110 in the manner described above. This immediately drops out the relay and shuts down the system until such time as it has had a chance to cool and the manual re-set button 119 has been depressed to reset the safety switch 29.

For installations in which automatic recycling is not desired or required, contact pairs 60 and 61 of stack switch 30 and elements 75-78, 80, and 92-95 of the stack switch clutch assembly may be simply omitted, and relay contact pair R2 may be returned directly to tap 133 on the secondary winding of transformer 27.

It is also. apparent that the primary control of the present invention is well adapted to the provision of a remote combustion detector switch, merely by enclosing combustion detector switch 30, the clutch mechanism of FIGURE 6, and the stack mounting elements and thermally sensitive device 65 in a separate housing. It is further apparent that this may be done with a recycling or non-recycling control. Moreover, all of these modifications may be provided with maximum standardization of parts and ease of installation. The time lag adjustment on the recycling mechanism permtis ready adaptation of the control to widely varying furnace installations. The housing construction and removable cover plates facilitate installation and wiring of the unit while maintaining the required safety precautions, and the removable cover plate 44 and the construction of the housing and the chassis facilitate access to and removal of the control components for repair or service without disconnecting the electrical conduit or fittings. In actual practice, a defective control may be serviced by removing the entire chassis member 25 with all of its operating components 8 "and replacing it with a complete unit in operating condition, so that the defective unit may be serviced and repaired at a remote service center without disabling the furnace installation.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

' 1. In a fuel burner primary control of the type comprising a relay and a series control circuit for energizing said relay, a thermally responsive safety device including: a pair of mating contacts included in said series control circuit; a thermally sensitive bi-metallic element for normally maintaining said contacts in closed condition; and means consisting essentially of a resistance element having a substantial positive temperature coefficient of resistance included in said series control circuit and responsive to the sustained flow of electric current therein for progressively reducing the energizing voltage impressed on said relay until the latter becomes de-energized, said bi-meta-llic element being in juxtaposition with said resistance element to be heated thereby to release said contacts to an open circuit condition subsequent to such de-energization of said relay.

2. In a fuel burner control system, in combination: an ignition device responsive to the flow of electric current for igniting said burner; means including a control relay comprising a pair of normally open contacts in series with said ignition device and further comprising an energizing solenoid for closing said contacts to actuate said ignition device; and a series control circuit including a source of electric current, said solenoid, and a thermally responsive safety device consisting essentially of a resistance element having a substantial positive temperature coefiicient of resistance, said resistance element being entirely responsive to the sustained flow of electric current therein for progressively reducing the energizing voltage impressed on said solenoid until the latter becomes de-energized to reopen said contacts and de-actuate said ignition device.

References Cited by the Examiner UNITED STATES PATENTS 1,485,580 3/ 1924 Zimmerman ZOO-116.8 1,858,265 5/1932 Dahlstrom 158-28 1,948,938 2/ 1934 Lawton 200-122 2,046,962 7 1936 Mott 317-22 2,231,686 2/1941 Shaw ZOO-138.1 2,233,018 2/1941 Lindemann ZOO-138.1 2,237,262 4/ 1941 Miller 200-122 2,292,837 8/1942 Jackson 200-168 2,293,158 8/1942 Merkel 200-168 2,340,502 2/1944 Baker 317-41 2,476,330 7/ 1949 Sitzer 317-41 2,482,820 9/1949 Wolfson et al 317-41 2,528,591 11/1950 Getchell 200-122 2,610,273 9/1952 Judson 200-116.1 2,740,923 4/1956 Loeber -1 317-132 2,758,175 8/1956 Hotchkiss 200-122 2,807,758 9/1957 Pinckaers 317-132 3,114,082 12/1963 Weise 317-132 SAMUEL BERNSTEIN, Primary Examiner.

RICHARD M. WOOD, Examiner. 

1. IN A FUEL BURNER PRIMARY CONTROL OF THE TYPE COMPRISING A RELAY AND A SERIES CONTROL CIRCUIT FOR ENERGIZING SAID RELAY, A THERMALLY RESPONSIVE SAFETY DEVICE INCLUDING: A PAIR OF MATING CONTACTS INCLUDED IN SAID SERIES CONTROL CIRCUIT; A THERMALLY SENSITIVE BI-METALLIC ELEMENT FOR NORMALLY MAINTAINING SAID CONTACTS IN CLOSED CONDITION; AND MEANS CONSISTING ESSENTIALLY OF A RESISTANCE ELEMENT HAVING A SUBSTANTIAL POSITIVE TEMPERATURE COEFFICIENT OF RESISTANCE INCLUDED IN SAID SERIES CONTROL CIRCUIT AND RESPONSIVE TO THE SUSTAINED FLOW OF ELECTRIC CURRENT THEREIN FOR PROGRESSIVELY REDUCING THE ENERGIZING VOLTAGE IMPRESSED ON SAID RELAY UNTIL THE LATTER BECOMES DE-ENERGIZED, SAID BI-METALLIC ELEMENT BEING IN JUXTAPOSITION WITH SAID RESISTANCE ELEMENT TO BE HEATED THEREBY TO RELEASE SAID CONTACTS TO AN OPEN CIRCUIT CONDITION SUBSEQUENT OT SUCH DE-ENERGIZATION OF SAID RELAY. 